1) Field of the Invention
This invention relates to an optical pickup device for reproducing signals recorded on, for example, a magneto-optical recording medium.
2) Prior Art
The method for detecting signals of a magneto-optical recording medium, such as a magneto-optical disc, consists in detecting the information recorded as the direction of magnetization on a signal recording surface of the magneto-optical disc after converting it into minute rotation of a polarization plane by the Faraday effect or the Kerr effect which is the interaction between the light and magnetization. However, since the pin photo diode etc. taking charge of photoelectric conversion lacks in selectivity with respect to polarized beam, there is employed a method of detecting the rotation of the polarization plane after converting it into changes in beam intensity using a polarized beam splitter or a polarizer.
3) Description of the Prior Art
Referring to FIG. 1, a conventional signal detecting device for a magneto-optical disc is explained. In this figure, the laser beam from a laser beam source 21, such as a semiconductor laser device, is a laser beam of a P polarization component (P wave). This laser beam is irradiated on the signal recording surface of the magneto-optical disc 9 by an objective lens 20 via a beam splitter 22 so as to be converged on the signal recording surface. The light reflected from the magneto-optical disc 9 is again incident on the beam splitter 22 and again reflected by the beam splitter 22. The reflected beam from the beam splitter 22 has its polarization plane rotated by 45.degree. by a 1/2 wavelength plate 23 and is separated by a polarized beam splitter (PBS) 24 into polarized beam components having planes of polarization which are at right angles relative to each other. The first polarized light component reflected by the polarized beam splitter 24 is incident on a photodetector 25 constituted by a pin diode. The second polarized beam component transmitted through the polarized beam splitter 24 is incident on a photodetector 26 similarly constituted by a pin photo diode. Output signals of the photodetectors 25, 26 are inputted to a differential amplifier 27 by which in-phase components are removed. That is, the in-phase components are removed by an optical differential method, so that only signals of the magneto-optical disc, that is optical magnetic signals, are detected.
Signals recorded on a magneto-optical disc are usually detected by taking advantage of changes in intensities of the beam reflected from the optical disc. In such case, a servo control system, such as focusing and tracking control units, need to be provided in the optical pickup device, in addition to optical components for reading information signals from the optical disc, so that the laser beam from the laser beam source is focused on a signal recording surface of the disc while the laser beam correctly follows the recording track(s) of the disc. Random accessing may be achieved by feeding the optical pickup device radially of the disc. Above all, if faster random accessing is demanded of an optical disc recording and/or reproducing apparatus, the feed velocity of the optical pickup device becomes crucial. For achieving a fast feed of the optical pickup device, it is required of the optical pickup device to be of a small size and a small weight.
In order to meet this demand, a self-coupled optical pickup (SCOOP) making use of a self-coupling effect of a laser diode, is under investigation. FIG. 2 shows an arrangement of the self-coupled optical pickup which is required for signal detection of the optical recording medium, as taught by, for example, the Japanese Patent Publication No. 57-58735 (1982).
In FIG. 2, a laser diode 41 has two parallel cleavage end faces 41a, 41b, running at right angles to a pn junction surface 41j, which cleavage end faces constitute reflecting mirror surfaces to provide light resonation.
When a driving current I is injected from, for example, a driving circuit, laser oscillation is produced at higher than a threshold value of the injection current, so that a laser beam is radiated from the end face 41a. The laser beam is passed through a collimator lens 43 and an objective lens 44 and converged on a reflecting mirror M which may, for example, be a reflecting surface of an optical disc, such as its signal recording surface. The laser beam is then reflected by this reflecting mirror M and returned to the end face 41a of the laser diode 41 to constitute an external resonator.
If there is any return beam, the threshold value of the injected current is decreased substantially in proportion to the intensity of the return beam. If the value of the injected current is set at an intermediate value Im between the threshold value when there is the return beam and that when there is no return beam, a beam emitted from the laser diode 41 is significantly changed in conformity to changes in reflectance of the reflective mirror M. A rear beam emitted from another end face 41b of the laser diode 41 is also significantly changed in proportion to a light output from the end face 41a, so that, by providing a photodetector 42, such as a photodiode, facing the end face or terminal 41b, for detecting the rear beam emitted of the laser diode 41, an optical pickup which is small-sized and lightweight may be produced.
Meanwhile, the laser diode 41 and the photodetector 42 may be accommodated in one and the same package. The collimator lens 44 may also be omitted to provide a unitary lens system.
Although it is possible with the above-mentioned SCOOP to reduce the size and the weight and to lower the costs of the optical pickup device, there is raised a problem of a high signal noise level, above all, a problem of a noise caused by stimulated vibrations produced as a result of the distance between the beam source and the reflecting surface of the return beam and the beam velocity. The present Assignee has already proposed in JP-A-01 232544 a pulse read system in which, by setting the irradiation time duration of a laser beam intermitted at a predetermined period so as to be shorter than the basic period of the vibrations produced by the return beam, any noise produced by the stimulated vibrations may be inhibited to provide a SCOOP type optical pickup device of a practically useful level.
However, a differential optical system is still needed, as in the case of FIG. 1, if the SCOOP is to be used for reproducing the magneto-optical disc. Thus the above proposal has not been fully satisfactory in mitigating the complexity of the optical pickup device.
Furthermore, signals +.theta..sub.k and -.theta..sub.k are reproduced as magneto-optical (MO) signals of the magneto-optical recording medium. After reproduction, these signals are frequently detected by differentiation by an electric circuit for decoding as data. Since the differential detection is carried out on a time frequency scale, it becomes necessary to change the equalizer setting by the difference in MTF. For example, it is necessary to change the time constant depending on, for example, the linear velocity of the disc during reproduction or on the difference between the velocities on the inner and the outer peripheries.